Particulate rubber reinforcing carbon black is often used to reinforce various rubber compositions. In practice, a particulate carbon black is first formed into pellets comprised of the carbon black and binder to facilitate handling of the carbon black and particularly to reduce carbon dust in the rubber product manufacturing facility. The carbon black pellets are then blended with the rubber composition. As the rubber composition is being mixed under high shear conditions, the carbon black pellets disintegrate, into particulate rubber carbon black reinforcement within the rubber composition. Such use of pelletized rubber reinforcing carbon black for preparation of various rubber compositions is well known to those having skill in such art.
In practice, rubber reinforcing carbon black may be manufactured, for example, by forming fumed carbon black under controlled conditions at a significantly elevated temperature to form carbon black in a form of fine carbon black aggregates comprised of primary carbon black particles fused together in an uncompacted form of very small particle size which, while having a specific gravity in a range of from about 1.6 to about 1.9, depending upon the carbon black grade, but because the carbon black aggregates are separated by a significant amount of empty space (voids), they typically have a very low apparent density in a range, for example, from about 100 to about 250 kg/m3, and have an appearance of being relatively fluffy in nature and are therefore referred to herein as being “fluffy”.
The fluffy, fine particle sized carbon black aggregates are typically very dusty in nature and present associated handling problems in an industrial environment for manufacturing carbon black reinforced rubber products.
Accordingly, in order to make such fluffy carbon black aggregates transportable and useable in a form of a relatively non-dusting material, binder materials are conventionally used to glue the particulate carbon black aggregates together and to therefore significantly reduce the amount of empty space between the carbon black aggregates. The resulting composite of carbon black aggregates and binder material is typically pelletized to form pellets of the carbon black to promote the transportability of the composite. The apparent density of the carbon back aggregates may thereby be increased to perhaps an apparent density in a range, for example, of about 270 to about 600 kg/m3, depending somewhat upon the choice of carbon black type and binder material, because the voids between the carbon black aggregates are filled with the binder material.
The binder material physically interacts with the surface of the carbon black aggregates, binds the carbon black aggregates together and promotes a degree of hardness to the pellets.
In practice, such binder materials are typically organic material binders such as for example, molasses, soluble cellulose, methyl cellulose and soluble starch materials.
Intuitively, it is necessary that the composites of carbon black pellets are fragmented, or disintegrated, during high shear mixing of rubber compositions, to form the particulate fragmented carbon black reinforcement filler in situ within the elastomer host.
The efficiency of such fragmentation, or disintegration, of such carbon black pellet composites in situ within the elastomer host in the presence of a particulate precipitated silica and silica coupling agent is considered herein to be important to provide an appropriate high degree of dispersion of the carbon black fragments within the rubber composition to promote various appropriate physical properties of the rubber composition such as, for example, abrasion resistance and also sometimes tear resistance of the vulcanized rubber composition.
It is considered herein that the hardness of the carbon black pellets plays a role in such fragmentation, or disintegration, efficiency and resultant promotion of physical properties of the rubber composition, particularly for various components of a rubber tire such as, for example, a tire tread.
While the mechanism may not be completely understood, it appears that the hardness value of the pelletized rubber reinforcing carbon black composites is important where the reinforcing filler for the rubber composition is a combination of the fragmented rubber reinforcing carbon black and precipitated silica together with a silica coupling agent. Such phenomenon may be perhaps due, for example, to some kind of interaction, or quasi interaction, between the process of disintegration, or fragmentation, of the carbon black pellets in which their hardness value plays a part in the presence of the particulate precipitated silica and silica coupling agent which is preferentially reactive with hydroxyl groups on the precipitated silica, all in situ within the elastomer host or, alternatively or in combination, some kind of phase relationship or perhaps some kind of preferential affinity within the elastomer host of the elastomer for the fragmentation of the carbon black pellets in the presence of the particulate precipitated silica and silica coupling agent, in which the hardness of the carbon black pellets plays some kind of a roll in the efficiency of providing carbon black fragments, particularly where a good dispersion of the carbon black fragments within the elastomer host is desired.
Further, in another aspect, it is sometimes desired to prepare relatively soft uncured rubber compositions for various tire components, such as for example a high performance tire tread for which the rubber composition is mixed in an internal rubber mixer in which considerably reduced shear mixing conditions are used or experienced. In such case, the carbon black pellets might be inefficiently or even incompletely fragmented, or disintegrated, in situ within the rubber composition. This can result in a relatively poor carbon black dispersion within the elastomer host which may consequently lead to relatively poor various cured rubber physical properties for the rubber composition itself such as for example a relatively poor abrasion resistance. In such case, it may be desirable to provide the carbon black pellets with a reduced hardness, particularly where the carbon black pellets are to be fragmented in situ within the elastomer host in the presence of precipitated silica and silica coupler.
Accordingly, it is desired herein to prepare rubber compositions which contain a combination of precipitated silica, silica coupler and fragmented rubber reinforcing carbon black pellets by use of pelletized rubber reinforcing carbon black of a controlled hardness.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber.
The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated.
The term “fluffy” rubber reinforcing carbon black aggregates, where used herein, means such fluffy carbon black aggregates in an uncompacted form, typically with an apparent density in range, for example, of from about 100 to about 250 kg/m3.
The term “apparent density” where used herein means the ratio of the carbon black weight that fills a volume of one cubic meter (m3) according to ASTM D1513 Test Method for Carbon Black Pellet Pour Density.
The term “specific gravity” if used herein means the true carbon black material specific gravity according to ASTM D792-55 Test Method.